Process for producing methacrylic acid

ABSTRACT

A catalyst composition useful for the oxidation of unsaturated aldehydes, particularly the oxidation of methacrolein to produce methacrylic acid, comprises the combination of oxides of molybdenum, copper, phosphorus, antimony, cesium and rhenium in predetermined relative atomic ratios.

CROSS-REFERENCE TO OTHER APPLICATIONS

This is a division of application Ser. No. 047,860 filed June 12, 1979,now U.S. Pat. No. 4,377,501, which is a continuation-in-part ofapplication Ser. No. 973,354, filed Dec. 12, 1978, now U.S. Pat. No.4,374,757, which is a continuation-in-part of Ser. No. 972,745, filedDec. 26, 1978, now U.S. Pat. No. 4,252,682, which is acontinuation-in-part of application Ser. No. 973,495, filed Dec. 26,1978, now U.S. Pat. No. 4,252,683.

PRIOR ART

This invention relates to a process and catalyst for the vapor-phaseoxidation with molecular oxygen of methacrolein to methacrylic acid.

It is well known that unsaturated acids, such as acrylic acid andmethacrylic acid, can be produced by the vapor-phase oxidation of thecorresponding unsaturated aldehydes by means of molecular oxygen in thepresence of a suitable oxidation catalyst. A variety of catalystcompositions have been proposed for this purpose. Many such compositionscomprise the oxides of molybdenum and phosphorus in association with theoxides of various other elements, both metallic and non-metallic.

For example, and with respect to the catalyst to be discussed hereafter,British Pat. No. 1,430,337 and U.K. Pat. No. 4,000,088 propose the useof the catalyst composition in which the oxides of molybdenum andphosphorus are combined with the oxides of antimony, and copper andoptionally with chromium. The catalyst does not contain cesium orrhenium.

U.S. patents disclosing related catalysts which may contain cesiuminclude U.S. Pat. Nos. 4,051,179 and 4,042,533. in '179 copper andvanadium are treated as alternatives, while an alkali metal must beincluded, but antimony is considered optional. In '533 tungsten andvanadium are required, while copper and phosphorus are optional andantimony is lacking.

The catalyst disclosed in U.S. Pat. No. 4,042,533 also contains rheniumas an optional ingredient. Still another prior art patent disclosing theuse of rhenium as an optional ingredient is U.S. Pat. No. 3,956,378.While this catalyst requires the presence of molybdenum and antimony, itlacks cesium, and copper and phosphorus are only optional ingredients.

It has been found that catalysts for oxidation of methacrolein tomethacrylic acid have the characteristic property of remaining stablefor a long period of time and then, without warning, of beginning arapid decline in activity. Consequently, an increase in the useful lifeof such catalysts has been sought.

Despite the many disclosures of the prior art, an improved catalyst ofthis type is not developed merely by combining the many elements whichhave been disclosed. Instead the performance of a series of catalystcompositions is determined experimentally. Small changes in compositionmay be very important in achieving improved catalyst performance andparticularly in optimizing the catalyst composition to suit a specificreaction and set of operating conditions. The point is well illustratedby the improved catalyst formulation to be described hereinafter, inwhich the compositions of the parent disclosures of this applicationhave been revised to provide an improved composition shown to havesubstantially improved useful life.

SUMMARY OF THE INVENTION

It has been discovered that when using the catalysts to be described toproduce methacrylic acid by vapor phase oxidation of methacrolein, it ispossible to achieve both high activity and high selectivity for asignificantly improved useful life compared to previous catalysts andeven to the catalysts of the parent applications. The catalystcomposition comprises oxides of molybdenum, copper, phosphorus,antimony, cesium, and rhenium in predetermined relative atomic ratios.More specifically, the catalyst composition of the invention comprisesthe oxides of the above specified elements in the following atomicratios: Mo=12, Cu=0.05-3, P=0.1-5, Sb=0.01-1, Cs=0.1-3, andRe=0.005-0.5. Preferably, P will be 0.5-3. The catalyst composition maybe regarded either as a mixture of oxides of the named elements or asoxygen-containing compounds of the elements or both.

The catalyst composition used in the process of the invention also maybe expressed by the following general formula:

    Mo.sub.a Cu.sub.b P.sub.c Sb.sub.d Cs.sub.e Re.sub.f O.sub.g

wherein a to g indicate the atomic ratio of each component and, when ais 12, b is 0.05-3, c is 0.1-5, d is 0.01-1, e is 0.1-3, f is 0.005-0.5,and g has a value which is determined by the valence and proportions ofthe other elements in the catalyst. Preferably, c will be 0.5-3.

When such a catalyst as has been described is in contact with avapor-phase mixture of methacrolein, molecular oxygen, steam, andnitrogen at typical temperatures in the range of 250°-400° C. andpressures in the range of 0-5 atmospheres, excellent activity andselectivity to the production of methacrylic acid is obtained for alonger period of time than has been obtained heretofore.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE graphically displays the change of reaction temperatureover a period of operation to compare the useful life of oxidationcatalysts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Catalyst Composition andPreparation

The catalyst of the invention comprises oxides or oxygen-containingcompounds of molybdenum, copper, phosphorus, antimony, cesium, andrhenium in predetermined atomic ratios, as expressed in the followinggeneral formula: Mo_(a) Cu_(b) P_(c) Sb_(d) Cs_(e) Re_(f) O_(g) whereina to g indicate the atomic ratio of each component and, wherein a is 12,b is 0.05-3, c is 0.1-5, d is 0.01-1, e is 0.1-3, f is 0.005-05, g is avalue determined by the valence and proportions of the other elements inthe catalyst. Preferably, c will be 0.5-3. Other elements, which may beincluded in minor amounts in the catalyst formulation in order topromote catalyst activity or selectivity and without losing theadvantages to be shown for the general formula, are considered to bewithin the scope of the invention. The catalyst composition may beregarded either as a mixture of oxides of the named elements or asoxygen-containing compounds of the elements or both. As prepared and/orunder reaction conditions, the catalyst may contain either or both formsand both are intended to be included within the phrase "mixtures ofoxides".

It has now been discovered that the life of the catalysts disclosed inthe parent disclosures can be remarkably improved if cesium issubstituted for the calcium used in U.S. Ser. No. 973,495 and thephosphorus content is adjusted to a higher value than previouslyconsidered to be preferred. Alternatively, the catalyst may beconsidered to be the catalyst of U.S. Ser. No. 973,354, with rheniumadded. Preferably, the phosphorus content is higher than the preferredvalue of the parent disclosures. As will be seen in the subsequentexamples, such an improved composition has a much longer life whenoperated under severe conditions in an accelerated aging test.

The catalyst composition is preferably used in unsupported form, e.g. inthe form of pellets or other like compressed shapes of various sizes,although conventional supports could be employed instead. Thecomposition may be formed in conventional manner using techniques wellknown to persons skilled in the art. For example, compounds ofmolybdenum, copper, phosphorus, antimony, cesium, and rhenium aredissolved in a small amount of water or other solvent, and the solutionsare then combined and evaporated to dryness, e.g. in a rotary dryer. Theseveral components can be introduced into solution in the form ofvarious salts or other compounds of convenient types and no specificform for the catalyst precursors is necessary. The use of ammoniumsalts, halides, e.g. chlorides, nitrates or acid forms of the elements,e.g. phosphoric acid, are, however, particularly suitable. Preferably,however, aqueous solutions are employed and water-soluble forms of theelements are used. In some cases the solutions may have acids and/orbases added to them to facilitate dissolution of the catalystprecursors. For example, acids such as hydrochloric or nitric acid, orbases such as ammonium hydroxide, can be used as desired. The resultingpowder from the evaporation is then thoroughly dried and preferablyscreened to eliminate large particles which make it difficult to produceuniform compressed shapes, such as pellets. Typically, the powder ispassed through a 20-mesh screen. The powder is then mixed with anorganic binder which can be of any conventional type, such as polyvinylalcohol, and the mixture is thoroughly dried and again screened,typically to provide a 20-60 mesh size. The dried mixture is thenpreferably combined with a lubricant, again of any conventional type,such as stearic acid or graphite, and compressed into the desired shape,e.g. pelletized, the compressed shapes typically having heights anddiameters of 1/16 inch to 3/8 inch. Finally, the thus produced catalystcomposition is activated at high temperature for a prolonged period inaccordance with conventional practice in this art. For example, thepellets are placed in an oven or kiln, or in a tube through which air ispassed, at an elevated temperature (e.g. 300°-500° C., preferably325°-450° C.) for at least ten hours. In a particularly preferredactivation step, the temperature is raised at the rate of 20° C. perhour to a maximum of 420° C., preferably 320°-400° C., and thistemperature is maintained for 8 hours.

It will be understood that the foregoing description regardingpreparation of the catalyst in a form suitable for use in a vapor-phaseoxidation reaction is merely illustrative of many possible preparativemethods, although it is a particularly suitable method and is preferred.

METHODS OF OPERATION

The catalysts described are generally useful for the production ofunsaturated acids by oxidation with molecular oxygen of unsaturatedaldehydes, although the reaction of methacrolein to form methacrylicacid is of particular interest. Other possible starting materials arethe monoethylenically unsaturated aliphatic monoaldehydes of from 3 to 6carbon atoms, such as acrolein, crotonaldehyde, 2-methyl-2-butenal, andthe like, or mixtures thereof.

The reaction in which the catalyst compositions of this invention are ofparticular utility and in which they provide high conversions andselectivities involves contacting the catalyst with methacrolein andoxygen in the vapor phase, preferably also in the presence of steam anddiluents. When the catalyst of this invention is used in the vapor-phaseoxidation of methacrolein to form methacrylic acid, the oxidationconditions employed are those generally associated with this reaction,although it is preferred that the molar ratio of oxygen to methacroleinshould be kept at a high value near the flammable range. Once reactionis begun, it is self-sustaining because of its exothermic nature. Avariety of reactor types may be employed such as fluid or fixed bedtypes, but reactors having the catalyst disposed inside a multiplicityof heat exchanger tubes are particularly useful and convenient.

The gaseous feed to the reactor contains appropriate concentrations ofmethacrolein, oxygen and steam and usually an inert gas, such asnitrogen, is also present. The oxygen is usually added as such or asair, which may be enriched with oxygen. As mentioned, conventionaloxidation conditions can be employed but it is a feature of the catalystof this invention that methacrolein can be present in concentrations ofmore than 5 up to about 20 volume percent of the total feed with apreferred range of more than 5 up to about 15 volume percent. In generalat least 6 volume percent of the aldehyde is used in the feed. Thecorresponding ranges of oxygen are 3 to 15 volume percent, preferably 5to 12 volume percent and for steam up to 50 volume percent, preferably 5to 35 volume percent, the balance being the inert gas or gases.

The temperature of the reaction should, for best results, be within therange of from about 270° to 450° C., preferably 280°-400° C., and theoptimum temperature range is 290° to 325° C. Because the reaction isexothermic, means for conducting the heat away from the reactor arenormally employed to avoid a temperature increase which favors thedestruction of methacrolein by complete oxidation to carbon oxides andwater. The reactor temperature may be controlled by conventional methodssuch as by surrounding the catalyst-containing tubes with a molten saltbath.

The reaction may be conducted at atmospheric, superatmospheric orsubatmospheric pressure. Preferably, however, pressures are employedranging from atmospheric up to about 8 kg/cm² absolute, preferably up toabout 6.3 kg/cm² absolute, and most preferably up to about 4.5 kg/cm²absolute.

The unsaturated acid product may be recovered by a number of methodswell known to those skilled in the art. For example, the acid may becondensed, or scrubbed with water or other suitable solvents, followedby separation of the unsaturated acid product from the scrubbing liquid.The gases remaining after the acid-removal step may be recycled to thereaction preferably after removal of CO₂ by conventional means, e.g.,absorption in aqueous carbonate solution.

The features of the invention will be more readily apparent from thefollowing specific examples of typical catalyst preparation and its usein the oxidation of methacrolein. It will be understood, however, thatthese examples are for the purpose of illustration only and are not tobe interpreted as limiting the invention.

COMPARATIVE EXAMPLE EXAMPLE 1

In 750 cc of water are dissolved 636 grams of (NH₄)₆ Mo₇ O₂₄.4H₂ O. Then21.7 grams of Cu(NO₃)₂.3H₂ O are dissolved in 100 cc of water, 79.2grams of Ca(C₂ H₃ O₂)₂.XH₂ O are dissolved in 500 cc of water, 20.5grams of SbCl₃ are dissolved in a mixture of 30 cc of water, and 10 ccof concentrated HCl and 34.5 grams of H₃ PO₄ are dissolved in a mixtureof 100 cc of water. These solutions are fed to a rotary dryer of 4000 cccapacity and the mixture is evaporated to dryness at a temperaturereaching a maximum of 140°-200° C. The resulting powder is removed fromthe dryer and dried in an oven at 200° C. for 12 hours. The dried powderis screened through a 20-mesh screen, a 4% aqueous solution of polyvinylalcohol is added in sufficient quantity to make a damp mixture and thismixture is dried at 75°-80° C. until the moisture content falls to 2-4wt. %. The dried mixture is then screened to 20-60 mesh size particles,and about 2-6% of stearic acid powder is thoroughly mixed with it. Theresulting mixture is then pelletized to form pellets of 3/16 inch heightand diameter in which the catalyst components molybdenum, copper,phosphorus, antimony, and calcium are present (by calculation) in theatomic ratios of 12, 0.3, 1, 0.3 and 1.5, respectively. The pellets arethen activated in an oven by heating them to 100° C. in one hour andthen gradually at a rate of 20° C. per hour to 380° C. and maintainingthem at this temperature for 8 hours.

COMPARATIVE EXAMPLE EXAMPLE 2

A 150 cc quantity of the catalyst composition of Example 1 is placed ina reactor defined by a 1/2"×90" stainless steel pipe, the reactor pipebeing filled with 50 cc of inert filler (silicon carbide) below thecatalyst bed and 100 cc of the inert filler above the catalyst bed inconventional manner to insure uniform temperature contact with thecatalyst. Nitrogen-diluted mixtures containing methacrolein, oxygen andsteam are fed to the reactor at a pressure of 1.74 kg/cm² (absolute) andat a space velocity of about 1200 hr⁻¹. The term "space velocity" isused in its conventional sense to mean liters of gas (at standardtemperature and pressure) per liter of catalyst per hour. The feedcomposition is approximately, by volume, 6-7% methacrolein, 11-12%oxygen and 20% steam, the balance being nitrogen, determination beingmade on a wet basis. The reaction is run continuously and the exit gasis analyzed at intervals of several hours. Analyses are carried out bymeans of gas chromatography and by infrared spectrography usingconventional techniques. The average amount of methacrylic acid producedis determined periodically and the reactor temperature is adjusted asnecessary to obtain the desired yield, that is, the product of theconversion and the selectivity.

For comparison of many catalysts, all of which are capable of providinga satisfactory yield of methacrylic acid, but which differ in theiruseful life, an accelerated aging test is carried out on the catalyst ofExample 1 and reported in the sole FIGURE. The catalyst is tested undersevere conditions in order to obtain relatively quick determination ofthe catalyst performance. The operating temperature is raised to thelevel needed to achieve a predetermined yield of methacrylic acidequivalent to about 70-80% conversion of methacrolein to methacrylicacid with a selectivity to methacrylic acid of about 75-80%. Forcommercial operation, the temperature most suitable for obtaining thebest yield of methacrylic acid over a long period of useful catalystlife would be selected. As the catalyst deactivates, it is necessary toraise the operating temperature to maintain a constant methacrylic acidyield. The upper limit of catalyst temperature is reached when thetarget yield of methacrylic acid can no longer be obtained. This isgenerally found to be about 325°-330° C. At that point, the catalyst nolonger is considered satisfactory, although it retains some activity.While for commercial operation a useful life of at least 2-3000 hours isdesired, by means of the accelerated aging test, even durable catalystsmay be fully deactivated within a few hundred hours, thus providingcatalyst life information which might be obtained only after thousandsof hours under less severe conditions.

COMPARATIVE EXAMPLE EXAMPLE 3

A catalyst corresponding to that of Example 1 is prepared by the samegeneral technique except that 5 grams of Re₂ O₇ dissolved in 100 cc ofwater are included in the initial solution to provide rhenium in acatalyst having the following nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1.5 Re.sub.0.07 O.sub.g

The catalyst is tested under the conditions of Example 2 and the resultsplotted in the sole FIGURE, where it may be compared to the results ofExample 2.

While the use of calcium in itself provides a catalyst having improvedlife compared to the catalyst of Example 4 containing cesium instead ofcalcium, the addition of rhenium provides greatly increased useful life.While the catalyst containing calcium has lost essentially all of itsactivity in about 160 hours, and has an induction period of about 80hours, the catalyst containing rhenium in addition to calcium has auseful life of about 360 hours and an induction period of about 200hours.

COMPARATIVE EXAMPLE EXAMPLE 4

A catalyst corresponding to that of Example 1 is prepared by the sametechnique except that 58.4 grams of CsNO₃ dissolved in 150 cc of wateris substituted for the calcium acetate of Example 1 and thus a catalysthaving the following nominal composition (by calculation) is obtained:

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Cs.sub.1 O.sub.g

The catalyst is tested under the conditions of Example 2 and the resultsplotted in the sole FIGURE, where it may be compared to the results ofExamples 2 and 3. In the accelerated test, the catalyst containingcesium begins to lose activity after an induction time of only about 30hours and has a useful life of about 70 hours.

EXAMPLE 5

A catalyst was prepared according to the method of Example 3 but thecalcium was replaced by cesium and rhenium was included in the form ofperrhenic acid thereby providing a catalyst having the following nominalcomposition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.g

This catalyst is tested following the procedures of Example 2 and theresults shown in the sole FIGURE for comparison with the previousexamples. It will be evident that the catalyst has a longer life in theaccelerated test than a catalyst formulated without cesium (Example 4).Addition of rhenium thus is advantageous with a cesium containingcatalyst, as it was in catalysts containing calcium (see Examples 2 and3). However the catalyst containing cesium and rhenium is seen to have ashorter life in the accelerated test than the catalyst containingcalcium and rhenium.

It now has been discovered that the phosphorus content affects thesecatalysts remarkably, as will be shown in the following example.

EXAMPLE 6

A catalyst is prepared according to the method of Example 5 but theamount of phosphorus is doubled to provide a catalyst having thefollowing nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.2 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.g

The catalyst is tested according to the procedures of Example 2 and theresults shown in the FIGURE for comparison with the previous examples.It will be seen that the catalyst of Example 5 (where P is 1) maintainsits initial activity for about 80 hours and then begins to decline inactivity, as indicated by the increase in reactor temperature which isnecessary to maintain a constant level of methacrolein conversion. Inmarked contrast, the catalyst of this Example 6 appears to have a lowerinitial activity, as indicated by the higher temperature required toobtain the desired level of methacrolein conversion, but the catalystmaintains its initial activity for about 360 hours and then begins adecline in activity less steep than the catalyst of Example 5. Theuseful life of the catalyst of Example 6 is about three times longerthan that of the catalyst containing only half the amount of phosphorus(Example 5).

This remarkable improvement in catalyst performance may be contrastedwith the response of the catalyst of Example 3 when additionalphosphorus is included in the formulation, and as seen in the followingExample.

EXAMPLE 7

A catalyst corresponding to Example 3 is prepared with double the amountof phosphorus and the rhenium was included in the form of perrhenicacid, having the following nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.2 Sb.sub.0.3 Ca.sub.1.5 Re.sub.0.07 O.sub.g

The catalyst is tested under the conditions of Example 2. The resultsobtained for this catalyst cannot be plotted in the FIGURE since theoverall conversion of methacrolein is low and the selectivity tomethacrylic acid is unacceptable. Typically, for the catalysts havingtheir performance plotted on the FIGURE the conversion of methacroleinis about 70-80% and the selectivity to methacrylic acid is also about75-80%. For the catalyst of this Example 7, at an operating temperatureof 276° C., the conversion of methacrolein was about 26% and theselectivity was about 54%. It should be understood that the lowselectivity indicates that about half of the methacrolein is beingburned to carbon oxides and water, which requires so much of the oxygensupplied to the catalyst that the conversion of methacrolein isnecessarily very low when the oxygen supply is limited, as it is in theoxidation of methacrolein to methacrylic acid.

It is apparent that catalysts containing molybdenum, copper, antimony,rhenium, and alkali or alkaline earth metals are very sensitive to thephosphorus content. The apparent reversal of response of catalystscontaining cesium or calcium to increased phosphorus content, exhibitedin Examples 6 and 7, is unexpected and suggests that there will be anoptimum phosphorus content associated with each catalyst and which couldbe determined by routine experimentation. It is thought that increasingthe phosphorus content of the catalysts of the invention, that is thosecontaining molybdenum, copper, antimony, phosphorus, cesium and rhenium,that the catalyst performance eventually will decline. The results ofExamples 5 and 6 suggest that the optimum phosphorus content would beabove the level of one atom of phosphorus for each 12 atoms ofmolybdenum.

Another example of a catalyst according to the invention is given asfollows.

EXAMPLE 8

A catalyst is prepared according to the general method of Example 6except that the amount of phosphorus is lowered to produce a catalysthaving the following nominal composition (by calculation).

    Mo.sub.12 Cu.sub.0.3 P.sub.1.5 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.g

This catalyst is tested according to the methods of Example 6 and thecatalyst is found to have superior performance to the catalyst ofExample 6.

What is claimed is:
 1. A process for the preparation of methacrylic acidwhich comprises oxidizing methacrolein in the vapor-phase with molecularoxygen in the presence of a catalyst composition having the formulaMo_(a) Cu_(b) P_(c) Sb_(d) Cs_(e) Re_(f) O_(g), where: a=12; b=0.05-3;c=0.1-5; d=0.01-1; e=0.1-3; f=0.005-0.5; and g=value determined by thevalence and proportions of the other elements of the formula.
 2. Aprocess as defined in claim 1, where c=0.5-3.